Hens scratching at ears; head looks dingy & yellow-ish

I noticed one hen has a small bald spot on the side of her head. I watched to see what was going on and she does scratch at it but not frantically. The bald spot is the only clue that something's wrong.

So I threw some scratch around my feet to bring them all in for a look-see. A few hens look a little dingy on their heads, slightly yellow. Everyone acts fine. Eating, pecking, general chickenry - all well.

Perhaps coincidentally, my dog has also been scratching at her ears a bit. Do the same mites infect both chickens and dogs? Cats too? If so, will sevin dust treat the whole menagerie? Or is there something better? Our flock is small and fairly healthy so I don't have a lot of supplies on hand.

If the problem is not mites, maybe an infection or fungus, how will I know? And in what order should I go about this?

I do not know much about mites, but here is a post that I am sure could help you:http://www.backyardchickens.com/t/749736/mites-mites-mites
From what I understand, if it is mites you can see them crawling on the birds skin, especially around the birds vent. But I have heard of mites that burrow under the skin. Hopefully that link will help